1. Technical Field
The present invention relates to a non-toxic, lead-free solder alloy.
2. Related Art Statement
For many years, solder compositions which have been in frequent use include the metallic elements tin (Sn) and lead (Pb). Such alloys have been found to provide acceptable results in most environments. However, increased awareness of and concerns about problems of toxicity have resulted in lead no longer being an ingredient of choice.
It is anticipated that the federal government will regulate the use of lead in electronics packaging in the near future. The regulation may take one of the following possible scenarios: (1) increase taxes on Pb products, (2) classify Pb-containing products as hazardous materials and therefore require special handling and recycling efforts; and (3) ban the use of Pb in electronics packaging products.
In response to this situation, industries in U.S., Europe, and Japan are making efforts to find alternatives to Pb-bearing solders. Work is needed to develop, identify, acquire and evaluate Pb-free solders for use in automotive electronics products. In the electronics industry, for example, the solder interconnect is a critical part of electronics packaging, in which there are many applications requiring solder alloys that exhibit a high creep/fatigue strength, especially where soldered parts are subjected to mechanical stress over extended periods. Efforts which have been made include the development of Pb-free solder alloys, and product evaluation for applications in hybrid (thick film) products as well as printed wiring boards (PWB).
Consequently, there has developed an interest in finding a satisfactory way to eliminate lead from solder compositions and substitute other elements to yield equivalent or enhanced properties. In the automotive electronics field, a need has arisen to replace the currently used Pb-Sn base solders. Other toxic elements, such as Sb, Cd, etc., should also be minimized or excluded from consideration.
Most lead-free solder alloys are alloys which either contain other toxic constituents or have various other drawbacks and are therefore unsuitable for automotive electronics packaging applications. Representative of references which disclose such alloys are U.S. Pat. Nos. 4,806,309; 1,565,115; 4,797,328; 3,607,253; 3,157,473; and 3,503,721. Table A summarizes and compares the ingredients of compositions disclosed in these references, together with their melting temperature:
TABLE A __________________________________________________________________________ Melting Sn In Bi Ag Temperature U.S. Pat. No. [Wt %] [Wt %] [Wt %] [Wt %] Other [Wt %] [.degree.C.] __________________________________________________________________________ #1 4,806,309 90-95 -- 1-4.5 0.1-0.5 3-5 Sb 218-235 #2 1,565,115 87-92 -- -- 8-13 3 Au, Pd 283-330 #3 4,797,328 (1) 86-99 0-10 -- 0-13 1-10 Ti, Zr 700-950 (2) -- 0-10 -- 0-13 1-10 Ti, Zr . . . -- 86-99 Pb (3) -- -- 1-6 1-10 Ti . . . 86-93 Pb (4) -- 4-10 -- -- 1-10 Ti . . . 86-92 Pb (5) -- -- -- -- 1-10 Ti . . . 90-99 Pb #4 3,607,253 89.4-95.1 -- 1-5 3-3.8 0.7-1.3 Cd 210 (?) 0.2-0.5 Sb #5 3,157,473 (1) -- 100 -- -- -- 157 (2) -- 97 -- 3 -- 144 (3) 49.1 50.9 -- -- -- 120 #6 3,503,721 90-98 -- -- 2-10 220 __________________________________________________________________________
The '309 reference discloses a composition having antimony (Sb), which is essential to the functionality of the, alloy, but is toxic. The '115 reference discloses a composition with a melting temperature in the range of 283.degree.-330.degree. C., which is too high for general electronics packaging applications. Such environments generally require a melting point range of about 175.degree.-220.degree. C. The '328 reference discloses five compositions. All of those compositions have working temperatures in the range of 700.degree.-950.degree. C., which is prohibitively high for electronics packaging. Such alloys are primarily designed to bond ceramic articles. They contain Pb as their main constituent.
The '253 reference discloses cadmium (Cd) and Sb, which are essential for the functionality of the alloy, but are toxic. The '473 reference discloses three alloys. Each of them has a melting temperature which is too low for automotive electronics packaging applications. Such alloys may be useful for domestic products, however, which may be assembled with alloys having a lower operating temperature. The '721 reference discloses an alloy that was designed for dip soldering, which has limited applicability in the automotive electronics field. The current technology uses Pb-bearing solders, and Sn-Bi, which contains no lead but has a melting point of 138.degree. C., and is unsuitable for electronic packages with high power dissipation.
Against this background, there remains an unsatisfied need for lead-free, non-toxic soldering alloys which have a fine, stable microstructure, and thereby exhibit enhanced resistance to creep and fatigue. Preferably, such alloys should exhibit a melting temperature in the range of 175.degree.-230.degree. C. so that they may be used in high power devices which dissipate heat. Particularly in the automotive electronics industry, there is a need for new alloys which are designed to be used as a replacement for the currently used Sn-Pb solders in thick film and printed wire board products so that no other changes in manufacturing facilities or processes are needed.